programming timers that would remember with power failing

joeparrish · Jan 8, 2010

Ok here is the issuse chiller system that doses 3 chemicals
1 every 36hrs of run time

so chem 1 at 36 hours
chem 2 at 72 hours
chem 3 at 98 hours

then starts over after 36 hrs

this is done with a micrologics 1500 just by timer done bits turning on second and thrid timers

but the issue is if we lose power
we you get power back the program starts at the 1st chem

Is there a way to make it know what chem got dosed last if that makes sense

The only thing I can think of is a back up power supply

LadderLogic · Jan 8, 2010

joeparrish said:
The only thing I can think of is a back up power supply

Or a PLC that has an RTC (real-time clock) feature built in.

jstolaruk · Jan 8, 2010

Logic would have to be added to constantly store each timer's accumulated value (or use RTOs) and probably some latched bits to tell where in the process it was at anytime power was lost so that the process can be recovered on startup.

OkiePC · Jan 8, 2010

When the PLC loses power, do you want to have those timers stop and feeze at their current values?

If so, you can use the RTO instead of the TON timer. RTO timers will retain their accumulated values through a power cycle, and during the false state of the rung on which they're located. They must be reset with the RES instruction, since making the rung false simply "pauses" them.

If you need to keep timing when the power is off, then the real time clock is the way to go.

The choice will likely depend on whether or not the dosing pumps/valves stop when the PLC is powered off.

joeparrish · Jan 8, 2010

Yes the doseing pumps stop when the power is off.

Will a rto time with no power or just stop

Like if the timer is at 722 will it be 722 when the power is back on?

OkiePC said:
When the PLC loses power, do you want to have those timers stop and feeze at their current values?

If so, you can use the RTO instead of the TON timer. RTO timers will retain their accumulated values through a power cycle, and during the false state of the rung on which they're located. They must be reset with the RES instruction, since making the rung false simply "pauses" them.

If you need to keep timing when the power is off, then the real time clock is the way to go.

The choice will likely depend on whether or not the dosing pumps/valves stop when the PLC is powered off.

OkiePC · Jan 8, 2010

joeparrish said:
Yes the doseing pumps stop when the power is off.

Will a rto time with no power or just stop

Like if the timer is at 722 will it be 722 when the power is back on?

Yes. The timer.acc will hold its value when the rung goes false, and when the power is removed. It will not advance until the power is restored AND the rung is true. At that point, it will resume timing where it left off. When the timer .acc reaches the preset, it will stop advancing, and set the done bit. The only way to reset the RTO is with the reset instruction (RES).

Paul

programming timers that would remember with power failing

joeparrish

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OkiePC

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joeparrish

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